Geochemical variations in Cenozoic back-arc basalts at the border of La Pampa and Mendoza provinces, Argentina

Back-arc volcanism was active in central-western Argentina (provinces of Mendoza and La Pampa) from the Miocene through historic times. The rocks of 39 monogenetic volcanoes located in this area were studied in order to define their geochemical characteristics. The dominant rock texture is porphyritic, with intergranular, pilotaxitic and hyalophitic groundmasses. The most frequent phenocrysts are olivine followed by olivine–plagioclase–clinopyroxene. Their SiO2 content varies between 42.3 and 51.2 wt.%, the most abundant rocks are trachybasalts, followed by basalts and basanites, all of them alkaline. The rocks display enrichment of incompatible elements that varies according to the geographic location and age. There is an increase in incompatible element concentrations from the southern and central to the northern zones. Also, in the northern part of the study area, the behavior of incompatible elements varies with time; the incompatible element ratios of the Plio–Pleistocene rocks show arc signature, while the rocks of the Miocene De la Laguna volcano show intraplate affinity. We conclude for this sector that the mantle source region was modified after the generation of Miocene magmas by subduction-related fluids. These fluids are related to a late Miocene episode of subhorizontal subduction, i.e., after the generation of the rocks of De la Laguna volcano.